Resistance coupled amplifier



Sept. 5, 1933. v w, VAN B. ROBERTS 1,925,340

RESISTANCE COUPLED AMPLIFIER I Filed March 29, 1929 WALTER VAN B. ROBERTS 351* his www Patented Sept. 5, 1933 UNITED STATES RESISTANCE COUPLED AMPLIFIER 'Walter van B. Roberts, Princeton, N. J., assignor to Radiol Corporation of America, a corporation of Delaware l Application March 29, 1929.

6 Claims.

My present invention relates to couplings for amplifier stages, and, more particularly, to a method of, and means for, resistively couplingV are not basically different from the well-known form of low-frequency amplifiers of the same type diagrammatically shown in Figure 1 of the drawing. However, it must be observed that the resistance-repeating amplifier suffers from the fact that at radio frequencies the condensive reactance of the grid-filament circuit becomes so low asto practically short-circuit the' repeating resistance, r and consequently reduce the amplifying action. 1f Thus, a resistance-repeating amplifier which operates very successfully at audio frequency may fail to amplify at all at radio frequency, not because of any fault of the amplifier, but because of the short-circuiting action of the capacity between the grid and filament of each tube.

Now, Ihave discovered a method of eliminating the detrimental effects of the tube interelectrode capacities shunted across the repeating resistance which consists in replacing the repeat- 2 ing resistance by a filter system, lthetube inter- 'electrodecapacities being employed as a capacity across the filter input terminals, the resulting combination havingran impedance which functions as a pure and fairly constant resistance over a predetermined range'cf frequencies.

Accordingly, it is one of the main objects of the present invention to provide a resistancecoupledamplif-ier'including a filter member between each pair of amplifier tubes, the input terminals of each member v being connected to a capacity comprising the tube inter-electrode capacities of the aforesaid tubes whereby the filter member functions as a pure resistance over a predetermined range of frequencies.

Another important object of the invention is to provide a resistance-coupled amplifier-for radio frequency amplification which includes a filter member between each pair of amplifier tubes,

each filter having a capacity, comprising the vplate-filanflent capacity of the tube preceding it and the grid-filament capacity of the tube following it, across its input terminals, each filter member thereby functioning as a pure and fairly constant resistance over a predetermined range of frequencies. v j Thev novel features ,which 1 believe tobe characteristic of-my inventionareset forth in particularity in the appended'clainis, the invention t itself, however, as to'both itsorganization and CII Cil

method of operatonrwillbest be'understood by Serial No. 350,949

reference to the following description taken in connection with the drawing in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect. Y 60l t In the drawing,

Figure 1 shows diagrammatically a conventional typeof resistance-coupled amplifier circuit,

Figure 2 shows diagrammatically 4the amplifier circuit of Figure 1, with my invention applied thereto,

Figure 3 is a circuit diagram of a modified form of the coupling for the amplifier shown in Figure 2,

Figure 4 is a detail of the circuit shownk in 79 Figure 3 including a simplified form of thecoupling shown in the latter figure,

Figure 5 is a detail of an amplifier circuit including a coupling member comprising a band pass filter,

Figure 6 shows diagrammatically the amplifier circuit shown in Figure 1, with another modified form of coupling applied thereto.

`Referring to the accompanying drawing in which like characters of reference indicate the 80 same parts in the different figures, in Figure 1 there is shown a type of resistance-coupled amplifier, well known to those skilled inthe art, which is particularly adapted for audio-frequency amplification, and including two stages of amplification comprising tubes 1, 2, of the screen-grid type, it being understood that any other type of electron discharge tubemay be employed, the screen'element of the tubes being positively biased by sources S. i

The antenna circuit A is grounded at G and ytuned by a selector, the input circuit of tube 1 being coupled to the latter. The output circuit of tube l is coupled to the input circuit of tube 2 by a resistance R1 connected at one end to the plate of tube l, and at the other end to the positive terminal of a source B. A grid condenser Cgpis inserted in the gridcircuit of the tube 2,

a grid leak r2 being connected between 4the grid of tubeZ and the filament of the latter. 'Ihe 100 output circuit of the tube 2 is coupled to any Well known type of utilization means, suchl as phones, loud speaker, etc.

Experience has shown that a simple resistance coupling, such as shown in Figure 1, is useless at radiov frequencies due to thek short-circuiting effects of tube capacities effectively in shunt to the repeating resistance, as Ri, such tube capacities being designated as the capacityv Ci and shown `in Figure l by dotted lines. The detri- 5110 mental effect of the capacity Ci may be eliminated, and this capacity utilized to advantage by employing it in the type of circuit shown in Figure 2 for radio frequency amplification. The latter incorporates such capacitive reactance in a filter construction interposed as a coupling between each pair of stages of amplification.

In Figure 2, I have shown an amplifier circuit which illustrates the principles of the present invention, the circuit embodying the tubes 1, 2

preferably of the screen-grid type, the input circuit of the tube 1 being connected to the selector in the antenna circuit, as shown in Figure 1, and designated as Signal Input in Figure 2,

The plate circuit of tube 1 is connected through a grid condenser C2 to the grid of the subsequent tube 2. A grid leak r2 is connected from the filament to the grid of the tube 2, the output circuit of the tube 2 being coupled to any well known type of utilization means. The input terminals of a filter member F are connected to the positive terminal of a source B and the plate circuit of tube 1.

If only a few filter sections are to be used, a terminating resistance, well known in the art, may be used and is designated by Ri. The function of the terminating resistance is to cause the filter F to act approximately as if it was constructed of an indefinite number of sections as far as its input impedance is concerned.

It will be observed that the filter member F is so arranged relative to the tube l that the tube capacities Ci are connected across the input terminals of the filter member, and the filter member is so constructed that it has an iinpedance which acts as a pure and fairly constant resistance over a predetermined range of frequencies. The invention includes, then, emn ploying the tube capacities of each stage of amplification as the capacity (or portion of the capacity) required across the filter input terminals.

In Figure 3, I have shown an amplifier circuit, similar to that shown in Figure 2, in which the filter member F comprises a low pass filter having two meshes or sections, the first capacity consisting of tube capacities Ci effectively in shunt with the plate-filament circuit of tube l, and including, of course, the grid-filament capacity of the subsequent tube 2. The inductances Li, L2, and capacities C3, C4 are so chosen as to make the input impedance of the filter as high and uniform as possible over the range of frequencies to be amplified.

The voltage built up on the plate of the tube 1 is transferred to the grid of the subsequent tube 2 by a grid capacity C2, and the grid of tube 2 is maintained at a negative bias through a grid leak r2, as shown in Figure l. The terminal resistance Rt is shown, as in Figure 2, connected to the filter sections as explained heretofore.

In Figure 4, there is shown a stage of amplification coupled to a subsequent stage (not shown) by the filter member of the type shown in Figure 3, but simplified in that only one mesh or section is employed, the coupling being analogous in structuie and operation to that shown in Figure 3 in every way.

A low pass iilter is not the only coupling construction that can be employed between each pair of amplification stages. Figure 5 shows a band pass lter utilized as a coupling between the tube 1,V and the subsequent stage of amplification (not shown), it being observed that the tube y the effective inter-electrode capacities of the coupled tubes, and that the filter member have a suitably high, and preferably uniform, Impedance throughout the desired frequency range.

It will be obvious that the utilization of a band pass filter coupling between each pair vof stages involves the further advantage of discrimination against frequencies lying outside the desired band.

In Figures 6 I have shown an amplifier circuit, analogous to that shown in Figures 1, 2, 3 and 5 in which the grid of the subsequent tube 2 is coupled to the terminating impedance Rt, the latter then functioning simultaneously as al terminating resistance and a grid leak. In such a case, the grid-filament capacity is not effectively in shunt to the filter input, but to its output terminals.

It is to be observed that one of the input ter minals of the filter coupling F is connected to the positive terminal of the source B and that one of the output terminals of the filter is also connected to the positive terminal of the latter source, or the filament source. In this latter type of circuit, it is to be understood that the filter F may be a high, low, oi band pass type of filter member.

I have illustrated each stage of ampliiication as being of the screen-grid type of tube, but it is to be understood that if this type of tube is not used, resistance coupling increases the apparent grid-filament capacity of the tubes in accordance with well known formul. In that case the filters must be designed to operate with this apparent value of terminal capacity rather than the simple tube capacity alone.

While I have indicated and described only a few systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications in the circuit arrangements, as Well' as in the apparatus ein' v ployed, may be used without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

l. A radio frequency amplifier comprising a pair of electron discharge tubes, means for iinpressing radio frequency currents upon the input circuit of one of the tubes, a condensive pathV of low impedance to such currents directly'connecting the anode of said last tube to the control electrode of the other tube, a source of potential for the said anode,` a filter having a proper terminating resistor and presenting' a substantially constant pure resistance as its iinpedance Iover a considerable range of radio frequencies, connected between said source and said anode, alternating current potential differences produced across said filter throughout said range being impressed upon said loW impedance path.

2. A radio frequency amplifier comprising a i'io pair of electron discharge tubes, means for impressing radio frequency currents upon the input circuit of one of the tubes, a condenser of low impedance to such currents directly connecting the anode of said last tube to the control electrode of the other tube, a source of potential for the said anode, a filter having a proper terminating resistor, and presenting a substantially constant pure resistance as its impedance over a desired range of radio frequencies, connected between said source and said anode, alternating current potential differences produced across said filter through said range being impressed upon said condenser, said iilter including as an element thereof the inter-electrode capacities of both tubes.

3. A radio frequency amplifier comprising a pair of electron discharge tubes, means for impressing radio frequency currents upon the input circuit of one of the tubes, a condenser directly connecting the anode of Vsaid last tube to the control electrode of the other tube, a source of potential for the said anode, a properly terminated filter path, presenting as its impedance over a y desired range of radio frequencies a substantially constant pure resistance, connected between said source and said anode, alternating current po-l tential differences produced across said filter path throughout said range being impressed upon said condenser.

4. A radio frequency ampliiier comprising a pairof electron discharge tubes, means for impressing radio frequency currents upon the input circuit of one of the tubes, a condensive path of low impedance to such currents directly connecting the anode of said last tube to the control electrode of the other tube, a source of potential for the said anode, a low pass filter path having a proper terminating resistance, and presenting as its impedance a substantially constant pure resistance over a desired range of radio frequencies to be amplified, connected between said source and said anode, alternating current potential differences produced across said low pass filter path throughout said range being impressed upon said low impedance path.

5. A radio frequency amplifier comprising a pair of electron discharge tubes, means for impressing radio frequency currents upon the input circuit of one of the tubes, a condensive path of low impedance to such currents directly connecting the anode of said last tube to the control electrode of the other tube, a source of potential for the said anode, a band pass filter path including a proper terminating resistor, and presenting a substantially constant pure resistance as its imf lproper terminating resistor connected across the filter output, the coupling means presenting a substantially constant pure resistance as its impedance over the amplifier operating range of frequencies, a source of potential for the anode of the preceding tube, said lter being connected between the anode and said source.

WALTER VAN B. ROBERTS. 

